1. Field of the Invention
The present invention relates to a voltage selection circuit that selects one voltage from a plurality of voltages, and in particular, to a voltage selection circuit integrated in a semiconductor integrated circuit device.
2. Description of the Related Art
A semiconductor integrated circuit device, such as a CMOS, has a problem in that an excessive current flows to the inside through a parasitic diode of a transistor or the like when a reverse-polarity voltage is applied to power supply terminals (+terminal, −terminal) due to incorrect connection or the like. Japanese Unexamined Patent Application Publication No. 05-152526 discloses a protection circuit for preventing such reverse connection of a power supply.
In the protection circuit for preventing the reverse connection of a power supply disclosed in Japanese Unexamined Patent Application Publication No. 05-152526, protection against reverse connection is performed only for two lines of power supply terminals (+terminal, −terminal). However, for example, many semiconductor integrated circuit devices having a sensing function include not only power supply terminals but also terminals for signal input and output. In this case, since there is a possibility of incorrect connection among three lines of the power supply terminals (+terminal, −terminal) and the signal terminals, protection against the incorrect connection is not sufficient just by using the circuit disclosed in Japanese Unexamined Patent Application Publication No. 05-152526.
FIG. 23 is a diagram showing an example of a semiconductor integrated circuit device having a function of protecting an internal circuit against incorrect connection among three lines of power supply terminals VDD and VSS and an output terminal VOUT. The semiconductor integrated circuit device shown in FIG. 23 includes a circuit 100 having a function of a power switch for turning on or off the connection between the power supply terminal VDD and the output terminal OUT according to the detection result of a sensor or the like and a circuit 101 formed by three diodes having respective cathodes connected in common.
The circuit 101 is a circuit for supplying the highest voltage of the power supply voltage VDD, the ground potential VSS, and the output voltage VOUT to the circuit 100 as a power supply voltage. The configuration of the circuit 101 is the same as a rectifier circuit that has been used in a power supply device. Regardless of the relative magnitude relationship among the power supply voltage VDD and the ground potential VSS input from the outside and the output voltage VOUT, the highest voltage of these is always supplied to a power supply line on the high voltage side of the circuit 100. Accordingly, the power supply line is never lower than the ground potential VSS. In addition, since a P-type MOS transistor 111 having a bulk to which the highest voltage is applied is connected to the output terminal VOUT, no current flows to the bulk from the output terminal VOUT through a parasitic diode of the P-type MOS transistor 111. Therefore, according to the semiconductor integrated circuit device shown in FIG. 23, no excessive current flows to the internal circuit even if there is incorrect connection among the three lines of the power supply terminals VDD and VSS and the output terminal VOUT. Although the circuit 100 having a power switch function has been mentioned as an example in FIG. 23, the same protection as described above can also be realized for a circuit having a power regulator function or a signal input and output function.
However, since the circuit 101 shown in FIG. 23 is formed by diodes, a voltage drop in the forward direction inevitably occurs. This voltage drop is approximately 0.6 V in the case of a PN junction diode, and changes depending on the passing current or temperature. In recent years, the power supply voltage has become increasingly lower according to a reduction in the processing size in the semiconductor manufacturing process. If the power supply voltage is further reduced by such a voltage drop and the power supply voltage changes according to the current or temperature, a problem that the operation of a circuit becomes unstable occurs. In addition, there is also a problem that electric power is wasted and heat is generated if there is a voltage drop in a diode.